This project is directed at characterizing the involvement of light stimulation in the loss of discs from the tip of photoreceptor outer segments. Using in vivo and in vitro preparations analyzed with light microscopy, electron microscopy, autoradiography and cytochemistry, it will determine whether the detachment of discs from the outer segment tip in rods is a function of the photoreceptor or the pigment epithelium (PE). It will determine the subcellular mechanism involved in loss of the outer segment tip and will assess whether cone outer segment shedding is stimulated by light. It will also evaluate the mechanism of transport of disc debris to the lysosomal system within the PE.